1. Field of the Invention
The invention relates in general to microwave resonators and in particular to techniques for compensating for thermally-induced dimensional variations in such resonators.
2. Description of the Prior Art
An important objective of current satellite-developmental efforts is an increase in net transmission power. A long-standing obstacle to the achievement of this objective has been the unavailability of practical means for dissipating, especially from the associated narrow-band, microwave tuning elements, the significant amounts of heat which are inherently generated in high-power operation.
The readily-available nature of materials such as aluminum which do in fact possess high coefficients of thermal conductivity and which hence would otherwise be capable of dissipating the generated heat has in the past not facilitated the overcoming of this obstacle. Because such materials tend to inherently possess a complementary high coefficient of thermal expansion, the resulting lack of dimensional stability has generally made them unacceptable for use in the fabrication of tuned elements in which frequency stability is an essential consideration.
The primacy of the frequency-stability requirement has caused the typical prior-art practice to be restricted to the use of temperature-invariant materials such as Invar, graphite composites and quartz, which while dimensionally stable, inherently possess poor heat-dissipation characteristics. The net result is that in restricted environments such as satellites, transmitter operation has usually been limited to low-power levels.
It is accordingly a primary aim of the present invention to facilitate the realization of a significant increase in available transmitter power levels by providing a stabilized mechanism for increasing the power-handling capability of the associated narrow-band tuned elements.
The prior-art approach has also been disadvantageous in that the typically-utilized dimensionally-stable materials are costly and possess structural characteristics which are not conducive to ease of manufacture. Furthermore, these prior-utilized materials are relatively heavy, an especially-undesirable property in the inherently-weight-sensitive satellite environment.
It is also an aim of this invention, therefore, to provide thermally-stabilized microwave tuned elements which are economical, readily-manufactured and light-weight.
To the extent that the prior materials have in fact been utilized for high-power operation, such operation has typically required the employment of cumbersome supplemental structures. These structures have included such expedients as heat-sink elements or even pressurized assemblies with forced-air circulation. Arrangements of this nature have been additionally disadvantageous for a number of reasons. First, even though the employment of such units has enabled satellite power levels to be increased beyond the typical 5-to-20-watt range, the increase has usually been into the vicinity of only 40 watts at best and even this has represented an extension to the limits of the utilized-materials' operational properties. What is desired in contrast is an increase into the 100-to-200 -watt range. Second, and in addition to giving rise to still-higher levels of cost, manufacturing complexity and net weight, the more-involved nature of the resulting overall mechanisms has further compounded the difficulties associated with attempting to either cascade the tuned elements or otherwise configure them so as to minimize net operational interference. Also inherently associated with this higher level of complexity has been a correspondingly-decreased level of net reliability.
It is a further aim of this invention, therefore, to provide thermally-stabilized, high-power microwave tuned elements of inherent structural simplicity, combinatorial facility and operational reliability.
It may be noted that because the concept of an efficient, economical and lightweight means for the dimensionally-stabilized thermal compensation of narrow-band tuned microwave elements is not limited in applicability to satellite environments, the present invention provides a means for generally decreasing the net operational complexity of such elements in other environments as well.
Another important objective of current satellite development is the provision of microwave tuning elements which are substantially dimensionally stable despite thermal changes in the satellite's environment. Dimensional stability is important because the dimensions of a microwave tuning element such as a microwave cavity substantially determine its frequency response. One earlier approach toward the achievement of dimensional stability is illustrated in the drawings of FIG. 3. This earlier approach provides a thermally invariant bar to substantially hold end plates disposed on opposite ends of a microwave cavity in substantially fixed positions relative to one another to thereby substantially prevent thermally induced variations in the longitudinal dimensions of the microwave cavity.